The present invention relates generally to rubber compositions having improved thermal stability and green strength. These rubber compositions comprise blends of a first rubber component, an isoolefin/para-alkylstyrene copolymer rubber component and an amine.
Most conventional rubbers must be crosslinked, i.e., cured or vulcanized, in order to obtain suitably strong, shaped articles. However, crosslinking introduces relatively permanent bonds among the rubber""s polymer molecules. Once these bonds are formed, they prevent adequate flowability for subsequent processing or molding. Thus, it is often desirable to process and mold rubber articles prior to curing.
Prior to being crosslinked, however, these rubbers often lack sufficient strength, also known as green strength, for easy handling and processing. Green strength refers to the cohesiveness and dimensional stability under relatively low stress of a rubber compound before it is vulcanized or cured. Green strength is important in such industries as tire manufacturing, where the innerlining and other layers of the tire may be formed in one stage, while curing takes place in a later stage. Green strength is important in allowing the manufacturer to shape and stretch the uncured rubber, while maintaining other favorable properties such as low air permeability.
Ionomerization (i.e., ionomer formation) provides a means for modifying green strength without chemical crosslinking through a cure process. Ionomerization of amines is also known in the art as quaternization, due to the formation of a quaternary amine. Since it is a reversible process, ionomerization provides green strength at lower temperature as well as good processability at higher temperatures. A typical method of ionomerization is the addition of tertiary amines to the rubber compositions, the basic amine functionality interacting with a more acidic functional group on the polymer in order to form stable interactions that increase green strength. U.S. Pat. Nos. 3,898,253 and 4,256,857 generally describe the use of tertiary amines to directly modify halobutyl rubber compositions to improve their green strength. Halobutyl rubber, such as bromobutyl rubber, however, is not very reactive with these amines. Therefore, the mixture must be heated for a substantial period of time in order to obtain sufficient green strength.
The present inventors have found that rubber green strength and/or thermal stability is greatly improved by blending the rubber with an amine and an isoolefin/para-alkylstyrene copolymer. The amine preferentially reacts with the isoolefin/paralkylstyrene to form a quaternary amine ionomer which results in improved green strength. The ionomerization can be performed in situ and with little or no heating. It has also been discovered that these compositions, when cured, exhibit improved thermal and aging stabilization. Alternatively, depending on the type of amine used, stability may be improved without the formation of ionomers.
The present invention is a composition and a method of forming a composition blend, the composition blend comprising at least two components. A third rubber component may additionally be present in the blend of the invention. The first component is a halogenated isoolefin/para-alkylstyrene copolymer, hereinafter referred to as xe2x80x9ccomponent Axe2x80x9d. Component A is present in the blend from about 95% to about 5% by weight of the blend.
In one embodiment, the isoolefin has between 4 and 7 carbon atoms and said copolymer containing from about 0.5% to about 20% by weight para-alkylstyrene, wherein from about 0.01 mole % to about 60 mole % of the methyl groups present on the benzene ring of the para-alkylstyrene contain a halogen atom. In a preferred embodiment, component A is an EXXPRO(trademark) Elastomer (ExxonMobil Chemical).
The second component is an amine compound represented by the formula (R1R2R3)N, wherein R1 is either hydrogen or a C4 to C30 hydrocarbyl group, R2 is either hydrogen or a C1 to C30 hydrocarbyl group; and R3 is either hydrogen or a C1 to C30 hydrocarbyl group provided that at least one of R1, R2 and R3 is not hydrogen. The amine is present from about 0.1 to about 5% by weight of the blend. In another embodiment, the blend contains the amine from about 0.1 mole % to about 60 mole % relative to the mole % of halogen atom present in the blend. Further, the amine compound is selected from the group consisting of N,N-dimethyl hexadecylamine, N,N-dimethyl hexylamine, N,N-dimethyl dodecylamine, N,N-dimethyl octadecylamine and N,N-diethyl decylamine, N,N-dimethylbenzylamine, N,N-methyl propyl hexadecylamine, and morpholine.
When present, a third component is a rubber component (hereinafter referred to as a xe2x80x9crubberxe2x80x9d or xe2x80x9crubber componentxe2x80x9d) comprising from about 5% to about 95% by weight of the blend. The rubber is selected from the group consisting of butyl rubber, halogenated butyl rubber, isobutylene homopolymers, neoprene, nitrile rubbers, ethylene/propylene/diene terpolymers, ethylene/propylene copolymers, an isoolefin/para-alkylstyrene, a halogenated isoolefin/para-alkylstyrene, and natural rubbers.
In another embodiment, the rubber component is a halogenated butyl rubber, wherein the halogenated butyl rubber contains from about 85% to about 99.5% by weight repeat units derived from isobutylene, from about 0.1 to about 15% by weight repeat units derived from conjugated diene and from about 0.1% to about 15% by weight halogen. In a preferred embodiment, the conjugated diene is derived from isoprene. In another preferred embodiment, the rubber component is a halogenated isoolefin/para-alkylstyrene.
The blends of the present invention comprise at least two components: a first halogenated isobutylene/paralkylstyrene component and an amine component. In another embodiment, another rubber may be present as a third component. The invention also includes a method of forming the blend with the amine and other components. The blend is initially uncured (or unvulcanized) when combined, but may also be cured to form a cured blend.
The rubber component is the constituent for which improved green strength is desired. The term xe2x80x9crubberxe2x80x9d or xe2x80x9crubber componentxe2x80x9d as used herein may include, but is not in any way limited to, the following polymers: butyl rubber, halogenated butyl rubber, isobutylene homopolymer, neoprene, nitrile rubbers, ethylene/propylene/diene terpolymers, ethylene/propylene copolymers, styrene butadiene rubbers, polybutadiene, polyisoprene, isoolefin/paralkylstyrene copolymer, halogenated isoolefin/paralkylstyrene copolymer, natural rubber, and mixtures thereof.
As used herein the term xe2x80x9cbutyl rubberxe2x80x9d is defined to mean a polymer predominately comprised of repeat units of isobutylene but including a few repeat units of a conjugated diene. Preferably from about 85% to about 99.5% by weight of the butyl rubber are repeat units derived from the polymerization of isobutylene, while from about 0.1% to about 15% by weight of the repeat units are derived from a conjugated diene having from 4 to 8 carbon atoms such as butadiene, isoprene, hexadiene, etc., with isoprene being preferred.
xe2x80x9cHalogenated butyl rubberxe2x80x9d is defined to mean butyl rubber that contains at least 0.05% by weight halogen such as chlorine or bromine, preferably bromine.
Preferred halogenated butyl rubbers are those that contain from about 0.1% to about 15% by weight halogen, more preferably from about 0.5% to about to about 10.0% by weight halogen based on the total weight of the halogenated polymer.
Numerous patents disclose halogenated butyl rubber containing various amounts of chemically bound halogen, see for example, U.S. Pat. Nos. 2,631,984; 2,732,354; 3,099,644; 2,732,354; 2,944,578; 3,943,664; 2,964,489; and 4,130,534 (each fully incorporated herein by reference).
As used herein xe2x80x9cnitrile rubbersxe2x80x9d are copolymers of acrylonitrile with a conjugated diene having from 4 to 8 carbon atoms, with butadiene being preferred.
As used herein, xe2x80x9cethylene/propylene copolymersxe2x80x9d are defined to mean those elastomeric or thermoplastic curable copolymers comprising ethylene and propylene. The preferred ethylene/propylene copolymer is one in which the ethylene component is between about 20 and 90% by weight of the copolymer.
As used herein, xe2x80x9cethylene/propylene/diene terpolymersxe2x80x9d are defined as those elastomeric or thermoplastic curable terpolymers comprising ethylene, propylene and diene units. Preferred diene units are 5-ethylidene norbornene, 5-methylidene norbornene, dicyclopentadiene, 1,4-hexadiene and 5-vinyl norbornene.
As used herein xe2x80x9cisoolefin/paralkylstyrene copolymersxe2x80x9d are those elastomeric or thermoplastic curable copolymers comprising isoolefin units, preferably isobutylene units, and a comonomer such as alkylstyrene, preferably para-alkylstyrene. Preferably, the isobutylene copolymers are characterized in that from about 85% to about 99.5% by weight of the polymer is derived from isobutylene. More preferably, the isobutylene copolymers are characterized in that from about 85% by weight to about 98% by weight, or more preferably from about 90% to 98% by weight, of the polymer is derived from isobutylene.
From about 0.5% to about 20% by weight, preferably from about 2% to about 15% by weight, or more preferably from about 2% to about 10% by weight, of the polymer is repeat units derived from alkylstyrene. Preferably, the alkyl group has from 1 to 10 carbon atoms, such as methylstyrene, and most preferably is para-methylstyrene. These polymers may also be halogenated so that they contain from abut 0.1% to about 5% by weight halogen, preferably from about 0.5% to about 2.0% by weight halogen based on the total weight of the halogenated polymer. Preferably the halogen is bromine. The preferred bromine species is benzyl bromine.
Stated another way, preferred halogenated isoolefin/para-alkylstyrene copolymers are those wherein from about 0.1 mole % to about 60 mole %, more preferably from about 0.5 mole % to about 30 mole %, of the methyl groups present on the benzene ring of the para-alkylstyrene contain a halogen atom. Such polymers are described is U.S. Pat. No. 5,162,445 which is fully incorporated herein by reference.
Any amine may be used as the amine component as long as it is sufficiently compatible with the rubber component, and component A, and as long as permanent crosslinking is avoided. When improved green strength is desired, preferably, the amine component is one capable of ionomerizing the copolymer component. These amines may be represented by the formula (R1R2R3)N wherein R1 is either hydrogen or a C4 to C30 hydrocarbyl group, R2 is either hydrogen or a C1 to C30 hydrocarbyl group, and R3 is either hydrogen or a C1 to C30, preferably a C1 to C8, hydrocarbyl group, provided that at least one of R1, R2 and R3 is not hydrogen. Preferably, R3 is a methyl or ethyl group and one of R1 or R2 is a C6 to C20 hydrocarbyl group and the other is a methyl or ethyl group. The hydrocarbyl groups, independently, may be saturated, unsaturated, cyclic or aromatic.
Thermal and age stability may be obtained even if the amine selected does not form ionomers with the copolymer component. Such amines tend to be more hindered and may be represented by the formula (R1R2R3)N wherein R1 and R2 are independently a C4 to C30 hydrocarbyl group and R3 is either hydrogen or a C1 to C30 hydrocarbyl group.
Examples of suitable amine components include, but are not limited to: N,N-dimethyl hexadecylamine, N,N-dimethyl hexylamine, N,N-dimethyl dodecylamine, N,N-dimethyl octadecylamine, N,N-diethyl decylamine, N,N-dimethylbenzylamine, N,N-methyl propyl hexadecylamine, and morpholine.
The component A, amine, and when present, the rubber component, are combined in amounts effective to produce the desired improvement in green strength and/or stability. The precise concentrations of each component will thus depend on the specific components used. The relative amounts of rubber and amine will generally dictate the degree of ionomerization since the amine will preferentially react with the rubber. When a halogenated rubber component is used, for example, the preferred amine level is from about 0.05 to about 2 mole equivalents of amine per rubber or component A halogen, more preferably from about 0.1 to about 1 mole equivalent of amine per rubber or component A halogen.
The present invention is particularly effective when blends of component A and another rubber component are employed. The relative amounts of component A, amine, and when present, the rubber component, will depend on the particular rubber component used. Preferably, only so much amine and component A is used as is needed to improve green strength and/or stability to the desired degree. For example, the weight percent of the rubber component may vary from as little as about 5% to up to about 95% by weight of the blend, more preferably from about 10% to about 90% by weight of the blend, even more preferably from about 20% to about 80% by weight of the blend.
The method used to combine the at least two components is not critical as long as there is adequate dispersion of the amine and other components within the rubber. The ionomerization occurs in situ. Thus, any mixing device may be used. Preferably mixing is facilitated by heating the mixture from 50xc2x0 C. up to 200xc2x0 C., and preferably from 50xc2x0 C. up to 150xc2x0 C. in an internal mixer or rubber mill. Notably, it is not necessary to heat the mixture and/or mix for extended periods of time after mixing is achieved in order to obtain adequate ionomerization. Once the components are blended, additional mixing or heating time for reaction should not be necessary.
The mixing order is not critical. For convenience, the at least two components may be blended at one time. Alternatively, the rubber component and amine (and other components when present) may be blended first, followed by addition of component A. For some applications, it may be desirable to pre-blend the amine and rubber and then add the component A, or pre-blend the component A and rubber followed by the amine.
Various additives may be used in suitable amounts. For example, various reinforcing agents or fillers such as carbon black, clay, silica, talc, and the like may be combined with the blend at any point during production. Various colorants may be added such as titanium dioxide, carbon black, etc. Other additives include antioxidants, stabilizers, processing oils, lubricants, anti-static agents, waxes, flame retardants and plasticizers.
After component A, amine, and when present, the rubber, are combined, the composition having improved green strength may be used directly in molded, extruded or shaped articles. It may be necessary to heat the blend in order to obtain the necessary viscosity for molding.
The blended compositions may also be cured using conventional curing or vulcanizing agents. Examples include sulfur and sulfur vulcanizing agents; various organic peroxides such as benzoyl peroxide, dicumyl peroxide, 2,5 dimethyl-2,5 di(tertbutylperoxy)hexane, and 2,2xe2x80x2-bis(tertbutylperoxy)diisopropyl benzene; hydrosilation curing agents; metal oxides such as zinc oxide or magnesium oxide, or organic zinc salts such as zinc stearate; diamines; co-curing agents such as various maleimides; and the like; all as set forth in U.S. Pat. No. 5,073,597, hereby fully incorporated by reference. Moreover, various phenolic resins known to the art and to the literature can be utilized, as well as various phenol-formaldehyde resins as set forth in xe2x80x9cThe Chemistry of Phenol-Formaldehyde Resin Vulcanization of EPDM: Part I. Evidence for Methylene Crosslinks,xe2x80x9d by Martin Van Duin and Aniko Souphanthong, Rubber Chemistry and Technology, vol. 68, pp 717-727, 1995, hereby fully incorporated by reference.
The amount of the curing agent will generally vary depending upon the type utilized and especially the desired degree of cure, as is well recognized in the art. For example, the amount of sulfur is generally from about 1 to 5, and preferably from about 2 to about 3 parts by weight per 100 parts by weight of the blend. The amount of the peroxide curing agent is generally from about 0.1 to about 2.0 parts by weight, the amount of the phenolic curing resin is generally from about 2 to about 10 parts by weight, and the amount of the hindered amine is from about 0.1 to about 2 parts by weight, all based upon 100 parts by weight of the blend.
Whenever a halogenated butyl rubber or any halogenated rubber is utilized, a small amount of an alkaline earth oxide compound such as magnesium oxide is utilized in an amount of about 3 parts by weight or less, and desirably about 2 parts by weight or less for every 100 parts by weight of the halogenated rubber. The alkaline earth oxide is added to act as a scavenger of hydrogen halides.
Conventional catalysts (accelerators) can also be utilized such as those known to the art and to the literature. For example, suitable amounts of various Friedel-Crafts catalysts can be utilized such as stannous chloride, salicylic acid, para-toluene sulfonic acid, zinc chloride, and the like.
The amount of the curative and accelerators are such that a degree of cure, that is, at least 90%, desirably at least 95 or 96%, preferably at least 97% or 98% of the curable rubber is nonextractable in a suitable solvent for the particular rubber at 23xc2x0 C. Suitable solvents include xylene, cyclohexane, acetone, hexane, toluene, and the like.
It has surprisingly been found that, once the blended compositions are cured, they demonstrate improved thermal and aging stabilization when the composition of the present invention is cured. Thus, not only is green strength improved, the overall characteristics of the cured rubber product are improved.
The blended compositions of this invention have a wide variety of ultimate uses. For example, tubings, hoses, gaskets, diaphragms, tires, innerliners, films, bumpers, membranes, adhesives, innertubes and other items where rubbers or rubber-copolymer blends are employed.
The following examples are presented to illustrate the foregoing discussion. Although the examples may be directed to certain embodiments of the present invention, they are not to be viewed as limiting the invention in any specific respect.